The present invention relates to apparatus and methods for heating and, thereby, drying, a plurality of plate-like metal objects such as metal can lids, also known as “closures” or “ends”.
Closures for metal beverage containers are generally of a circular shape with a flanged perimeter called a curl. The closures may also be of a rectangular shape. The closures are usually made of aluminum or steel, and the curl is used in attaching the closure to a can body through a seaming operation. To aid the integrity of the seal that is formed between the can body and the closure, it is a common practice to apply a bead of sealant or adhesive (“compound”) within the curl of the can end during manufacture of the closure. Different types of coatings are also selectively or generally applied to can closures and can bodies for various other purposes as well, for example, to repair damaged coatings. For the purposes of the present description, coatings, sealants and adhesives are all considered to be “liquids” applied to a workpiece.
It is necessary in this manufacturing operation to cure or dry such liquids. It is known to dry can closures by infrared radiation, convection heating, or induction heating. An induction dryer, for example, typically includes a cabinet that supports a tube extending generally horizontally across the cabinet from one end to the other. The tube is larger in diameter than the can ends. An induction coil is wrapped around the tube. The ends move through the tube in a stacked relationship, that is, with abutting face-to-face contact with each other (“in-stick”). When a suitable electric current is passed through the coil, the metal can ends are inductively heated. The heat is transferred to the compound on the can ends by conduction from the heated metal. The compound is heated and water is driven off from the compound into the surrounding air.
Because of the close proximity of one end to another in the stick, it is desirable to have as much warm air as possible contact the ends, while they are in the dryer, to remove the water from the area around the can ends. In one prior art induction dryer, air is heated with an ambient air heater that is mounted externally to the cabinet, for example, on top of the cabinet. The air flows from the heater along a flexible external duct and is directed into an air box secured on the inlet wall of the cabinet, surrounding the inlet opening into the tube. Some of the air flows from the air box to atmosphere through an opening in the air box that admits the moving can ends from an external source. The remainder of the heated air flows from the air box into the tube, flowing in the direction of the moving can ends. The air that is forced into the tube flows out the outlet end of the tube at the opposite end wall of the cabinet, under the force of the air being forced in at the inlet end. The flow of heated air through the tube helps to remove the moisture that is driven off from the heated can ends in the tube, and thus promotes drying of the ends.
In the prior art induction dryer, a thermocouple is located at the outlet end of the tube. The thermocouple is mounted in the end wall of the cabinet, at the circumferential top of the outlet opening. As the can ends pass through the outlet opening, the thermocouple registers the temperature of the can ends. The thermocouple provides an electric output that is used by a controller for the dryer to help control the current in the induction coil and/or other factors in the heating apparatus.
The thermocouple is adjusted to touch the can ends. This engagement of the thermocouple with the can ends can create a jam point if the ends are not in perfect stick form. Also, the thermocouple bracket is subject to deformation which would move the thermocouple away from the stick, which would register a temperature fault, shutting down the system.
The stick is, preferably, constantly moving. However, jams may occur, or some other occurrence may prevent the can ends from moving smoothly through the dryer. The prior art dryer includes a wheel that is mounted at the inlet end of the dryer and that contacts the upper edges of the moving can ends. If the stick stops moving, the wheel stops rotating, and an appropriate output signal is provided to the controller for the dryer, alerting it that the stick is not moving.
At times the induction coil tube needs to be removed from the cabinet, for example, for maintenance or to replace the tube with a different diameter tube more suitable for drying can ends of a different diameter. In the prior art dryer, the tube ends are held in place in the cabinet end walls with split collar hubs. Each upper hub is loosened by removing four screws. The upper hub can then be lifted upward a little and the tube can be pulled out of the cabinet through one end wall or the other of the cabinet. This process requires clearing away any equipment, such as an upstacker or a separator, from the end of the cabinet, to clear space for pulling out the entire tube, which may be four to eight feet in length.